In the telecommunications art, tests are performed on equipments, such as telephone sets to determine their commerical acceptability. As part of such tests, it is necessary to measure and indicate the amplitudes of certain a.c. signals produced in operation by such equipments.
For example, a telephone set providing TOUCH-TONE.RTM. dialing has therein an oscillator unit responsive to pressing of each of certain keys on the key pad of the set to generate a different pair of high frequency and low frequency sinusoidal a.c. signals which are audible when converted into sound, each such high frequency signal being a selected one out of four such signals ranging in frequency from about 1150 Hz to about 1700 Hz, and each such low frequency signal being a selected one out of four such signals ranging in frequency from about 650 Hz to about 850 Hz. Thus, the oscillator unit is capable of generating a total of eight signals of different frequency, of which four are high frequency and four are low frequency, and from which there is theoretically available sixteen different pairs of high and low frequency signals of which twelve pairs are used in practice.
In order for the TOUCH-TONE.TM. telephone set being tested to be commercially acceptable, each of the eight a.c. signals which can so be generated by such oscillator unit must have an r.m.s. amplitude in the range between 300 and 650 millivolts, the size of the range between those two limits being 350 mv. One way used in the past to determine whether or not such signals meet that criterion has been to measure the voltages of such signals with a standard a.c. voltmeter, That technique is, however, disadvantageous in that the voltmeter is hard to read accurately and requires more time for a reading thereof than is desirable. As another shortcoming of such technique, in order to obtain indications of r.m.s. signal amplitudes within the 300-650 millivolt range plus allowing some leeway for indications of amplitudes near to but not falling squarely within that range, the scale used on the voltmeter must extend from 0 volts to above 650 millivolts (as, say, to 1024 millivolts), but the use of a scale of such large size as compared to the 350 mv. size of the range of interest decreases the measuring resolution of the instrument as compared to what it would be if the scale, which could be used thereon were to be about the same size as that of the range of signal amplitudes of interest.
Another technique used in the past for determining whether or not the a.c. signals from the mentioned oscillator unit fall within the acceptable 300-650 mv. range has been to convert such a.c. signals into d.c. signals of magnitudes proportional to the r.m.s. amplitudes of such a.c. signals, and to then feed those d.c. signals to a d.c. comparator unit which is set to provide a "yes" indication if the magnitude of a particular d.c. signal falls within a range corresponding to the acceptable 300-650 mv. range for the a.c. signals, but to otherwise provide a "no" indication. Such "go-no go" kind of indication does not, however, provide the more accurate information on the actual r.m.s. amplitudes of acceptable a.c. signals which is often needed to fully evaluate the performance of the set being tested.